Colorimeter

ABSTRACT

A colorimeter comprising a light source, an operational amplifier, a reference photoconductive cell connected between the input and output of the amplifier and a sample photoconductive cell connected to the input of the amplifier. The amplifier provides an output signal which is a function of the ratio of the amount of light falling on the reference photoconductive cell to the amount of light falling on the saMple photoconductive cell. The output of the amplifier is connected to a suitable measuring device such as a meter or recorder which provides a measurement of the output signal.

iinited States'Patent 16am et all.

[151 3,656,856 [451 Apr. W7

[54] (IOLORIMETER Scientific Specialties Ltd., Garden City, NY.

22 Filed: Mar. 16,1970

21 Appl.No.: 19,875

[73] Assignee:

[52] US. Cl ..356/206, 250/218, 330/59, 356/181 [51] InLCl. ..G0ln21/22'[5.8] Field of Search ..250/209, 218; 330/59; 356/181, 356/206, 246

[56] References Cited UNITED STATES PATENTS 3,544,798 12/1970 Topol..250/2l8 3,344,702 10/1967 Wood et al ..250/2l8 X Primary Examiner-R.L. Wibert Assistant Examiner-R. J. Webster Attorney-Kenyon and KenyonReilly Carr & Chapin [57] ABSTRACT 27 Claims, 9 Drawing Figures RECORDERPOWER SUPPLY PATENTEDAFR 18 1372 3,656,856

SHEET 10F 3 I 22 a w 34 36 28 37% EcoRoER METER J' I 1; 38

FIG.2

INVEN'IORS ARTHUR L. LEVY GEORGE M. KATZ ATTOR N EYS PATENTEDAPR 18 m23,656,856

sum 2 OF 3 39 4' M RECORDER INVEN'IORS ARTHUR L. LEVY GEORGE M. KATZATTORNEYS Pmmgwmmz 3,656,856 SHEET 30F 3 FIG] Flea

INVENTORS ARTHUR L. LEVY GEORGE M. KATZ I ATTORNEYS COLORIMETERBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to colorimeters and more particularly to a new and improvedcolorimeter which provides a direct readout of transmittance that isunaffected by light fluctuations.

2. Description of the Prior Art Colorimeters are known for indicatingthe amount of light reflected from or transmitted through an object, theindication being used to determine some characteristic of the object.For

example, colorimeters are widely used in the determination of theconcentration of a light absorbing material present in a transparentfluid'through which light has been transmitted. One such application isin quantitative chemical analysis and more particularly blood analysisto determine the concentration of a particular material in a solution.In such application, the material under test may inherently exhibitlight absorbing power at a specific wavelength, i.e., is colored, or thematerial may have to be treated with suitable reagents in order tosensitize it to colored light.

In general, in known colorimeters, suitable colored light is transmittedthrough a container or cuvette containing a solution of the materialunder test and the light transmitted through the solution is detected bya photosensitive device to which is connected a meter which measures theamount of light transmitted through the solution (transmittance). Thismeasurement, in turn, may be used to determine the concentration of thelight absorbing material under test. In one type of known colorimeter, asingle photovoltaic cell is used as the light detector. The use of asingle photovoltaic cell is disadvantageous in that a large light sourcemust be provided due to the insensitivity of such cells. This requiresthe use of a large power supply for the light source and a large andbulky condenser system. In addition, any fluctuation in the intensity ofthe light source will cause false readings of concentration, thusnecessitating the use of a heavy and bulky constant power supply, suchas a battery or constant voltage transformer for the light source.

Another known colorimeter, makes use of a pair of photovoltaic cellsconnected in a Wheatstone Bridge configuration, to mitigate lightfluctuations in the light source. One cell is a reference cell andreceives light directly from the light source whereas the second cell isa sample cell and receives light transmitted through the test sample. Ameter is connected across the output terminals of the Bridge andmeasures any imbalance caused by less light falling on the sample celldue to light absorption by the material under test. A variablepotentiometer connected in one of the bridge legs is then adjusted tonull the meter, the change in resistance being a measurement of thetransmittance of the test solution and consequently of the concentrationof the light absorbing material. Since photovoltaic cells are used inthis type of colorimeter, a large light source (and large power supplytherefor) and bulky optical condensing system must be provided in orderto activate the insensitive cells. In addition, a direct reading is notprovided and a reading is only obtained through manipulation of apotentiometer. The low output of the photovoltaic cells is alsodisadvantageous.

Although, it has been proposed to use photoconductive devices incolorimeters, where only a single photoconductor is used, it will besensitive to light fluctuations thus necessitating use of a constantpower source. Where the use of two photoconductive devices in a bridgeconfiguration has been proposed, the necessity of using a nullingpotentiometer or the like unnecessarily complicates the device and makesit inconvenient. In addition, the output of the bridge may not be useddirectly for use in conjunction with other devices such as computers orthe like.

OBJECTS OF THE INVENTION It is therefore an object of the presentinvention to provide a new and improved colorimeter which produces adirect readout of transmittance that is unaffected by lightfluctuations.

It is a further object of the present invention to provide a colorimeterwhich utilizes a low level light source and minimal optical system,which has a low level power supply and operates at a low heat level witha minimum warm-up time and which produces a high level output which maybe used directly with recorders, computers and the like.

It is still another object of the present invention to provide acompact, eflicient and simply operated colorimeter which may be used toprovide measurement of the transmittance or optical density of asolution contained in a single cuvette or provide continuous measurementof transmittance or optical density of a solution flowed through aconventional flow cell.

It is yet another object of the present invention to provide a new andimproved colorimeter which is readily adapted to provide differentialcolorimetric measurements or expanded scale measurements.

It is still a further object of the present invention to provide a newand improved colorimeter which is adapted to produce a read out ofreflected or transmitted light.

SUMMARY OF THE INVENTION These and other objects and advantages areprovided by the present invention which generally comprises anoperational amplifier having a reference photoconductive cell connectedbetween the input and output of the amplifier and a samplephotoconductive cell connected to the input of the amplifier. Accordingto one aspect of the invention light from a light source is reflectedfrom or transmitted through an object onto the sample cell and is causedto fall directly on the reference cell. The output from the amplifierprovides a direct measurement of reflectance or transmittance and may beindicated by a meter or recorder. According to another aspect of theinvention a direct readout of optical density may be obtained by meansof a logarithmic amplifier connected between the operational amplifierand meter or recorder.

According to still another aspect of the present invention, difierentialmeasurements of transmittance may be obtained by passing light from alight source through two cuvettes or flow cells having difl'erentconcentrations of solution respectively onto the reference cell and ontothe sample cell.

Scale expansion is provided according to another aspect of the inventionthrough calibration of the meter by means of cuvettes having solutionsof known concentrations.

BRIEF DESCRIPTIONS OF THE DRAWINGS Other and further objects andadvantages of the present invention will be evident from the followingdetailed description of the invention and from the drawings wherein:

FIG. 1 is a perspective view showing one physical arrangement of some ofthe components of the apparatus of the present invention;

FIG. 2 is a schematic diagram of one embodiment of the presentinvention;

FIG. 3 is a schematic diagram of another embodiment of the presentinvention;

FIGS. 4, 5 and 6 are diagrammatic views showing preferred arrangementsof the light source and photoconductive cells as used in the presentinvention when measuring transmittance;

FIG. 7 is a schematic diagram of yet another embodiment of the presentinvention;

FIG. 8 is a diagrammatic view showing one arrangement of the lightsource and photoconductive cells as used in the present invention whenmeasuring reflectance; and

FIG. 9 is one arrangement of the light source and photoconductive cellsfor use in a differential colorimeter according to 5 the presentinvention.

DESCRIPTION OF THE EMBODIMENTS Referring now more particularly to FIG.1, there is shown one apparatus embodying the present invention. Asshown, housing 10, supports in the upper portion thereof, a meter 12having an indicator l4 and a scale 16. Positioned within the lowerportion of housing is a light source 18 mounted on bracket 20 secured tohousing 10. Light source 18 may be any suitable miniature bulb whichemits light over a broad spectrum of wavelengths.

Mounted opposite light source 18 are reference photoconductive cell 22and sample photoconductive cell 24. Photoconductive cells 22 and 24 arephotosensitive devices having resistances which vary as a function ofthe amount of light impinging thereon. Cell 24 is shown as mounted atthe rear of fluid flow cell 26 having respective inlet and outlet tubes27 and 29 connected thereto. Flow cell 26 comprises an internal chamberthrough which fluid is flowed and has transparent front and rear wallsto allow transmission of light from source 18 to cell 24 through thechamber.

Not shown is a filter hereinafter to be described which is normallypositioned between source 18 and cell 26.

Referring now to FIG. 2, there is shown adapted one preferred embodimentof the present invention.

As shown, reference cell 22 is connected across the input and output ofoperational amplifier 28 and sample cell 24 is connected to the input ofamplifier 28. Operational amplifiers are well known in the art andgenerally comprise a high gain d.c. amplifier such as that shown anddescribed at pages 544 546 of Servomechanisms and Regulating SystemDesign, (2nd ed.) Vol. I by Harold Chestnut and Robert W. Mayer,published by John Wiley & Sons, Inc., New York.

Power supply 30 is connected to amplifier 28 and supplies power theretoand is also connected to variable resistor 32 having wiper arm 34. Aswitch 36 is connected between cell 24 and the input to amplifier. Meter12 having limiting resistor 37 in series therewith and recorder 38 areprovided to measure the output of amplifier 28. Any suitable meter orrecorder well known to those skilled in the art may be provided formeter 12 and recorder 38.

Referring now to FIGS. 4, 5 and 6 there are shown preferred arrangementsof cells 22 and 24 for measuring the transmittance of a fluid containinga light absorbant material whose concentration is to be determined. FIG.4 shows light source positioned on one side of filter 40. Filter 40preferably passes light of a very narrow band of selected wavelengths,the selected wavelength band being dependent upon the band of lightwhich is optimally absorbed by the material under test.

The light absorbant material whose concentration is to be determined iscontained in a transparent container or cuvette 42 as shown in FIG. 5 oris flowed through a flow cell 26 as shown in FIG. 6. The filtered lightfrom filter 40 falls directly on reference photoconductive cell 22 butis transmitted through the fluid in cuvette 42 or flow cell 26 before itfalls on photoconductive cell 24.

The operation of the embodiment of FIG. 2 using the arrangement of FIG.4 and 5 will now be described. With no cuvette 42 inserted, normallyopen switch 36 will be open and act as if a very high resistance wereapplied to the input of amplifier 28. The reading of meter 12 will bezero corresponding to zero transmission and zero illumination falling onsample cell 22. Cuvette 42 is now filled with a transparent solutionsuch as water, solvent or reagent with no light absorbing materialpresent in the solution. Cuvette 42 is caused to close switch 36 andwiper 34 of resistor 32 is adjusted so that meter 12 reads full scalerepresenting 100% transmission. Cuvette 42 is then replaced with anothercuvette 42 containing the light absorbant material in solution. Sincethe output from amplifier 28 is a function of the ratio of theresistance of cell 22 to the resistance of cell 24 which in turn is afunction of the amount of light falling on cell 22 to the amount oflight falling on cell 24, it will provide a direct measurement of thepercent transmittance of the solution under test. Meter 12 willtherefore read directly the percent transmittance of the solution incuvette 42. This reading may be then converted to the concentration ofthe light absorbing material in the solution by well known procedureswhich are dependent upon the material under test.

Recorder 38 will also record on a chart the value shown by meter 12.

Where as shown in FIG. 6, flow meter 26 is used, switch 36 must bemanually operated. In such case, the opening of switch 36 will produce azero reading on meter 12 as heretofore described. This may also beaccomplished by preventing light from falling on cells 22 and 24, byobstructing the light path from source 18. A clear solution is thenflowed through cell 26 and resistor 32 adjusted to obtain a lOO percentreading on the meter. If a solution containing light absorbant materialis now flowed through cell 26, meter 12 will give a reading of thepercent transmittance of the solution and recorder 38 will give acontinuous recording thereof.

Referring now to FIG. 3 there is shown another embodiment of the presentinvention. As shown, cell 22 is connected between the input and outputof amplifier 28 and cell 24 is connected to the input of amplifier 28through switch 36. A variable voltage is also applied as a second inputto amplifier 28 from power supply 30 by means of fixed resistor 41,variable resistor 39 and fixed resistor 43. Meter l2 and recorder 38 areconnected to the output of amplifier 28 by means of variable resistor44. Current limiting resistor 45 is also provided in series with meter12.-Cell 24 is also connected to supply 30 by means of a voltage dividernetwork comprising fixed resistors 46 and 48.

In operation, a cuvette 42 containing a clear solution is positioned infront of filter 40 and caused to close switch 36. Resistor 39 isadjusted so that the meter reads 0, which corresponds to 100 percenttransmission. Switch 36 is now opened and resistor 44 adjusted to readfull scale which represents 0 transmission. Cuvette 42 containing asolution of color absorbant material under test is now inserted to closeswitch 36. Meter 12 will now read the percent transmission which canalso be recorded by recorder 38.

Following are specific values of components which may be used in thecircuit of FIG. 3. These values are given by way of illustration onlyand are not to be considered limiting the scope of the presentinvention.

COMPONENT VALUES 22 photoconductive cell Clairex 905HL 24photoconductive cell Clairex 705HL 41 fixed resistor ohms 2.7K 39variable resistor ohms l.5l( 43 fixed resistor ohms 470K 44 variableresistor ohms SK 45 fixed resistor ohms LSK 46 fixed resistor ohms lK 48fixed resistor ohms 2.2K

Meter 12, recorder 38, power supply 30 and operational amplifier 28respectively may be of any suitable type or configuration well known tothose skilled in the art and therefore will not be described in detail.

Where it is desirable to have meter 12 and/or recorder 38 indicate anexpanded scale of values, the circuit of FIG. 3 may be utilized. If, forexample, it is desired to have a range of percent transmittancedisplayed over the full scale of meter 12, the circuit of FIG. 3 iscalibrated by first inserting a cuvette containing a solution with 100percent transmittance to close switch 36 and adjusting resistor 39 sothat the current through meter 12 is zero and the scale reads 100percent. A cuvette containing a solution with 80 percent transmittanceis then inserted and resistor 44 adjusted so that meter 12 indicatesfull scale which corresponds to 80 percent transmittance.

Referring now to FIG. 7 there is shown an embodiment of the presentinvention wherein optical density may be read directly on meter 12. Asshown, reference photoconductive cell 22 and sample photoconductive cell24 are respectively connected between the input and output of amplifier28 and to the input of amplifier 28. A logarithmic amplifier 50 isconnected to the output of amplifier 28 and produces an output signalwhich is the logarithm of the output signal from amplifier 28. If thesolution under test satisfies Beer's law, meter 12 will give a directreading which is proportional to the concentration of the solution.

In certain applications, it is desirable to measure the light reflectedfrom an object, such as a turbid fluid, inorder to determine somequality or characteristic thereof. The arrangement shown in FIG. 8 maybe used according to the present invention to provide a measurement ofreflected light or reflectance. As shown, a container 52 holding amaterial under test, has light from light source 18 filtered by filter40, reflected from the lower surface thereof onto sample photoconductivecell 24. Reference photoconductive cell 22 is also provided and detectsdirect light radiated from source 18 through filter 40. It will beunderstood that any suitable circuit arrangement such as those of FIGS.2 or 3 may be used to indicate reflectance.

Referring now to FIG. 9, there is shown an arrangement of cells 22 and24 which may be used in the colorimeter of the present invention fordetermining the difference in transmittance between two solutions. Asshown, cuvettes 60 and 62 containing solutions of light absorbantmaterial of different concentrations are respectively positioned infront of cells 22 and 24. Radiated light from light source 18 isfiltered by filter 40 and transmitted through cuvettes 60 and 62. Wherecells 22 and 24 are connected in a circuit according to the presentinvention, for example, in the circuits of FIGS. 2 or 3, the measuringdevice, such as meter 12 or recorder 38 will give an indication of thedifference in transmittance of the solutions contained in cuvettes 60and 62.

Although the embodiments of the present invention hereinabove have beengenerally described in the determination of the transmittance orreflectance of a solution of light absorbant material, it will beunderstood that the colorimeter of the present invention may be used inthe determination of transmittance of any translucent or transparentobject, whether solid or fluid such as gas or liquid, and of thereflectance of any object which reflects light, whether solid or fluid.

In addition, the term light" as used herein is not to be limited tovisible light, but also includes the spectrum of radiated energy whichmay be detected by photoconductive cells. For example, non-visibleradiant energy in the ultraviolet and infrared wavelengths are alsoconsidered to be within the term light."

It will also be understood that although in many applications it isdesirable that radiant energy of a specific wavelength or narrow band ofwavelengths be used, the colorimeter of the present invention may alsobe used in conjunction with radiant energy of a broad band ofwavelengths, for example with ordinary daylight. Any suitable lightsource may be used and where colored light is to be used, it may beobtained by using a light source which radiates the desired coloredlight or by using a source of white light and filtering such light toproduce the desired colored light. Generally, the photoconductive cellswill be chosen to be responsive to the desired light chosen.

According to the present invention, the output signal of the colorimetermay be used directly as the input to any suitable device such as themeter and recorder mentioned hereinabove or to devices such asconcentration computers, scale expanders and other measuring andcomputing devices.

What is claimed is:

1. A colorimeter for analyzing a given sample by producing an outputvoltage which is a direct measurement of the concentration of thesample, comprising:

a. container means for holding the sample to be analyzed;

b. a source of radiant energy positioned on one side of the sample to beanalyzed;

c. a reference photoconductive cell positioned'between the source ofradiant energy and the sample, said photoconductive cell having aresistance which varies as a function of the radiant energy impingingthereon;

d. a sample photoconductive cell positioned on the opposite side of thesample from the light source, said photoconductive cell having aresistance which varies as a function of r the light which reaches saidsample photoconductive cell after passing throughthe sample;

e. amplifier means having an inputand an output, said samplephotoconductive cell being electrically connected to the input of theamplifier means and said reference photoconductive cell beingelectrically connected across the input and output of the amplifiermeans, the output of said amplifier being a function of the ratio of theintensity of radiant energy directed onto said reference photoconductivecell to the intensity of radiant energy directed onto said samplephotoconductive cell;

f. measuring'means connected to the output of said amplifier means;

g. a filter positioned between the source of radiant energy on one sideof the filter and the reference photoconductive cell, sample containingmeans, and sample photoconductive cell on the opposite side of thefilter; and

h. a housing to contain the source of radiant energy, referencephotoconductive cell, sample containing means, sample photoconductivecell, amplifier means, measuring means and filter;

i. wherein the radiant energy from said source of radiant energy fallsdirectly on said reference photoconductive cell, but is transmittedthrough the sample to be analyzed before said radiant energy falls onsaid sample photoconductive cell.

2. The colorimeter of claim 1 wherein the amplifier means is a high gaindc operational amplifier.

3. The colorimeter of claim 1 wherein said measuring means is a meter.

4. The colorimeter of claim 1 further comprising a logarithmic amplifierwhich is connected between the output of said amplifier means and saidmeter to produce an output which is the logarithm of the output of saidamplifier means to give a direct reading of optical density on saidmeter.

5. The colorimeter of claim 1 wherein said measuring means is arecorder.

6. The colorimeter of claim 1 wherein said measuring means comprises ameter and a recorder, both being connected to the output of saidamplifier means.

7. The colorimeter of claim 6 further comprising a source of variablevoltage which is applied as an input to said amplifier means to enablean expanded scale of values to be displayed on said meter.

8. The colorimeter of claim 7 wherein said source of variable voltagecomprises a source of fixed voltage and variable resistance connectedacross said source of fixed voltage and connected to the input of saidamplifier means.

9. The colorimeter of claim 1 wherein the container means for holdingthe sample to be analyzed is a transparent cuvette.

10. The colorimeter of claim 9 further comprising a switch which ispositioned in the electrical circuit between the sample photoconductivecell and said amplifier means, said switch being normally open and saidswitch being adapted to be closed by said cuvette.

11. The colorimeter of claim 1 wherein the container means for holdingthe sample to be analyzed is a fluid flow cell.

12. A colorimeter for determining the quantitative difference intransmittance between two solutions, comprising:

a. two container means, one for holding each of the two solutions;

b. a source of radiant energy positioned on one side of the twosolutions to be analyzed;

c. a reference photoconductive cell positioned adjacent to the first ofthe two solutions to be analyzed and positioned on the opposite side ofsaid solution from the source of radiant energy, said photoconductivecell having a resistance which varies as a function of the radiantenergy impinging thereon;

. a sample photoconductive cell positioned adjacent to the second of thetwo solutions to be analyzed and positioned on the opposite side of saidsolution from said source of radiant energy, said photoconductive cellhaving a resistance which varies as a function of the radiant energyimpinging thereon:

e. amplifier means having an input and output, said samplephotoconductive cell being electrically connected to the input of saidamplifier means and said reference photoconductive cell beingelectrically connected across the input and output of said amplifiermeans;

f. a measuring means connected to the output of said amplifier toindicate the difference in transmittance of the two solutions:

g. a filter positioned between the source of radiant energy and the twosolutions to be analyzed; and

h. a housing to contain the source of radiant energy, the two samples tobe analyzed, the reference photoconductive cell, the samplephotoconductive cell, the amplifier means, the measuring means and thefilter;

i. wherein radiant energy from said source of radiant energy istransmitted through the first of the two solutions to be analyzed ontosaid reference photoconductive cell and radiant energy from said sourceof radiant energy is transmitted through the second of the two solutionsto be analyzed onto said sample photoconductive cell.

13. The colorimeter of claim 12 wherein the amplifier means is a highgain dc amplifier.

14. The colorimeter of claim 12 wherein the measuring means is a meter.

15. The colorimeter of claim 12 wherein a logarithmic amplifier isconnected to the output of said amplifier meansto produce which is thelogarithm of the output of said amplifier means to give a direct readingof optical density on said meter.

16. The colorimeter of claim 12 wherein said measuring means is arecorder.

17. The colorimeter of claim 12 wherein said measuring means comprises ameter and a recorder, both being connected to the output of saidamplifier means.

18. The colorimeter of claim 12 wherein the container means for holdingeach of the solutions is a transparent cuvette.

19. The colorimeter of claim 12 wherein the container means for holdingeach of the solutions is a fluid flow cell.

20. A colorimeter for analyzing a given sample by measuring the lightreflected from the sample, comprising:

a. container means for holding the sample to be analyzed;

b. a source of radiant energy positioned on one side of the sample to beanalyzed to emit a light on the sample;

c. a reference photoconductive cell positioned adjacent to the source ofradiant energy to receive radiant energy directly from said source, saidphotoconductive cell having a resistance which varies as a function ofthe light impinging thereon from the light source;

a sample photoconductive cell positioned adjacent to the source ofradiant energy and adjacent to the sample to be analyzed to receiveradiant energy reflected from the sample, said photoconductive cellhaving a resistance which varies as a function of the light whichreaches said sample photoconductive cell after passing through thesample;

e. amplifier means having an input and an output, the samplephotoconductive cell being electrically connected to the input of theamplifier means and the reference photoconductive cell beingelectrically connected across the input and output of the amplifiermeans, the output of said amplifier being a function of the ratio of theintensity of radiant energy directed onto said reference photoconductivecell to the intensity of radiant energy directed onto said samplehotoconductive cell; f. measuring means or measuring the output of saidamplifier means;

g. a filter positioned between the source of radiant energy on one sideof the filter and the reference photoconductive cell, sample containingmeans, and sample photoconductive cell on the opposite side of thefilter; and

h. a housing to contain the light source, reference photoconductivecell, sample containing means, amplifier means, measuring means andfilter.

21. The colorimeter of claim 20 wherein the amplifier means is a highgain dc operational amplifier.

22. The colorimeter of claim 20 wherein said measuring means is a meter.

23. The colorimeter of claim 20 wherein a logarithmic amplifier isconnected to the output of said amplifier means to produce an outputwhich is a logarithm of the output of said amplifier means to give adirect reading of optical density on said meter.

24. The colorimeter of claim 20 wherein said measuring means is arecorder.

25. The colorimeter of claim 20 wherein said measuring means comprises ameter and a recorder, both being connected to the output of saidamplifier means.

26. The colorimeter of claim 20 wherein the container means for holdingthe sample to be analyzed is a transparent cuvette.

27. The colorimeter of claim 20 wherein the container means for holdingthe sample to be analyzed is a fluid flow cell.

k i t UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.Dated April 18, 1972 Inventor(s) Katz et 8 It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

In the Abstract Line 7, "saMple" should be --sample-- Line 10, after "ameasurement of the output signal." insertz --In one embodiment a cuvettecontaining or a flow cell passing a solution of light absorbing materialis positioned between the light source and the sample photoconductivecell. The output from the operational amplifier is measured by themeasuring device as per cent transmittance of the solution. In amodification, a logarithmic amplifier is connected to the operationalamplifier and the output from the logarithmic amplifier is measured bythe measuring device as optical density. In another embodiment, cuvettescontaining solutions of different concentrations are respectivelypositioned between the light source and the reference and samplephotoconductive cells, the output from the amplifier providing adifferential measurement of transmittance.--

Column 3, line 2 "shown adapted" should be --schematically shown--Signed and sealed this 8th day of August 97 (SEAL) Attes EDWARDTLFLETCHEIRJR. ROBERT GUTTSCHALK Attesting Officer Commissioner ofPatents 'OPM PO-1OSO (10-69) USCOMM-DC 6037 fi-P69 u 5 GOVERNMENTPRINTING OFFICE: I969 0-366-334

1. A colorimeter for analyzing a given sample by producing an outputvoltage which is a direct measurement of the concentration of thesample, comprising: a. container means for holding the sample to beanalyzed; b. a source of radiant energy positioned on one side of thesample to be analyzed; c. a reference photoconductive cell positionedbetween the source of radiant energy and the sample, saidphotoconductive cell having a resistance which varies as a function ofthe radiant energy impinging thereon; d. a sample photoconductive cellpositioned on the opposite side of the sample from the light source,said photoconductive cell having a resistance which varies as a functionof the light which reaches said sample photoconductive cell afterpassing through the sample; e. amplifier means having an input and anoutput, said sample photoconductive cell being electrically connected tothe input of the amplifier means and said reference photoconductive cellbeing electrically connected across the input and output of theamplifier means, the output of said amplifier being a function of theratio of the intensity of radiant energy directed onto said referencephotoconductive cell to the intensity of radiant energy directed ontosaid sample photoconductive cell; f. measuring means connected to theoutput of said amplifier means; g. a filter positioned between thesource of radiant energy on one side of the filter and the referencephotoconductive cell, sample containing means, and samplephotoconductive cell on the opposite side of the filter; and h. ahousing to contain the source of radiant energy, referencephotoconductive cell, sample containing means, sample photoconductivecell, amplifier means, measuring means and filter; i. wherein theradiant energy from said source of radiant energy falls directly on saidreference photoconductive cell, but is transmitted through the sample tobe analyzed before said radiant energy falls on said samplephotoconductive cell.
 2. The colorimeter of claim 1 wherein theamplifier means is a high gain dc operational amplifier.
 3. Thecolorimeter of claim 1 wherein said measuring means is a meter.
 4. Thecolorimeter of claim 1 further comprising a logarithmic amplifier whichis connected between the output of said amplifier means and said meterto produce an output which is the logarithm of the output of saidamplifier means to give a direct reading of optical density on saidmeter.
 5. The colorimeter of claim 1 wherein said measuring means is arecorder.
 6. The colorimeter of claim 1 wherein said measuring meanscomprises a meter and a recorder, both being connected to the output ofsaid amplifier means.
 7. The colorimeter of claim 6 further comprising asource of variable voltage which is applied as an input to saidamplifier means to enable an expanded scale of values to be displayed onsaid meter.
 8. The colorimeter of claim 7 wherein said source ofvariable voltage comprises a source of fixed voltage and variableresistanCe connected across said source of fixed voltage and connectedto the input of said amplifier means.
 9. The colorimeter of claim 1wherein the container means for holding the sample to be analyzed is atransparent cuvette.
 10. The colorimeter of claim 9 further comprising aswitch which is positioned in the electrical circuit between the samplephotoconductive cell and said amplifier means, said switch beingnormally open and said switch being adapted to be closed by saidcuvette.
 11. The colorimeter of claim 1 wherein the container means forholding the sample to be analyzed is a fluid flow cell.
 12. Acolorimeter for determining the quantitative difference in transmittancebetween two solutions, comprising: a. two container means, one forholding each of the two solutions; b. a source of radiant energypositioned on one side of the two solutions to be analyzed; c. areference photoconductive cell positioned adjacent to the first of thetwo solutions to be analyzed and positioned on the opposite side of saidsolution from the source of radiant energy, said photoconductive cellhaving a resistance which varies as a function of the radiant energyimpinging thereon; d. a sample photoconductive cell positioned adjacentto the second of the two solutions to be analyzed and positioned on theopposite side of said solution from said source of radiant energy, saidphotoconductive cell having a resistance which varies as a function ofthe radiant energy impinging thereon: e. amplifier means having an inputand output, said sample photoconductive cell being electricallyconnected to the input of said amplifier means and said referencephotoconductive cell being electrically connected across the input andoutput of said amplifier means; f. a measuring means connected to theoutput of said amplifier to indicate the difference in transmittance ofthe two solutions: g. a filter positioned between the source of radiantenergy and the two solutions to be analyzed; and h. a housing to containthe source of radiant energy, the two samples to be analyzed, thereference photoconductive cell, the sample photoconductive cell, theamplifier means, the measuring means and the filter; i. wherein radiantenergy from said source of radiant energy is transmitted through thefirst of the two solutions to be analyzed onto said referencephotoconductive cell and radiant energy from said source of radiantenergy is transmitted through the second of the two solutions to beanalyzed onto said sample photoconductive cell.
 13. The colorimeter ofclaim 12 wherein the amplifier means is a high gain dc amplifier. 14.The colorimeter of claim 12 wherein the measuring means is a meter. 15.The colorimeter of claim 12 wherein a logarithmic amplifier is connectedto the output of said amplifier means to produce which is the logarithmof the output of said amplifier means to give a direct reading ofoptical density on said meter.
 16. The colorimeter of claim 12 whereinsaid measuring means is a recorder.
 17. The colorimeter of claim 12wherein said measuring means comprises a meter and a recorder, bothbeing connected to the output of said amplifier means.
 18. Thecolorimeter of claim 12 wherein the container means for holding each ofthe solutions is a transparent cuvette.
 19. The colorimeter of claim 12wherein the container means for holding each of the solutions is a fluidflow cell.
 20. A colorimeter for analyzing a given sample by measuringthe light reflected from the sample, comprising: a. container means forholding the sample to be analyzed; b. a source of radiant energypositioned on one side of the sample to be analyzed to emit a light onthe sample; c. a reference photoconductive cell positioned adjacent tothe source of radiant energy to receive radiant energy directly fromsaid source, said photoconductive cell having a resistance which variesas a function of the light impinging thereon from the light soUrce; d. asample photoconductive cell positioned adjacent to the source of radiantenergy and adjacent to the sample to be analyzed to receive radiantenergy reflected from the sample, said photoconductive cell having aresistance which varies as a function of the light which reaches saidsample photoconductive cell after passing through the sample; e.amplifier means having an input and an output, the samplephotoconductive cell being electrically connected to the input of theamplifier means and the reference photoconductive cell beingelectrically connected across the input and output of the amplifiermeans, the output of said amplifier being a function of the ratio of theintensity of radiant energy directed onto said reference photoconductivecell to the intensity of radiant energy directed onto said samplephotoconductive cell; f. measuring means for measuring the output ofsaid amplifier means; g. a filter positioned between the source ofradiant energy on one side of the filter and the referencephotoconductive cell, sample containing means, and samplephotoconductive cell on the opposite side of the filter; and h. ahousing to contain the light source, reference photoconductive cell,sample containing means, amplifier means, measuring means and filter.21. The colorimeter of claim 20 wherein the amplifier means is a highgain dc operational amplifier.
 22. The colorimeter of claim 20 whereinsaid measuring means is a meter.
 23. The colorimeter of claim 20 whereina logarithmic amplifier is connected to the output of said amplifiermeans to produce an output which is a logarithm of the output of saidamplifier means to give a direct reading of optical density on saidmeter.
 24. The colorimeter of claim 20 wherein said measuring means is arecorder.
 25. The colorimeter of claim 20 wherein said measuring meanscomprises a meter and a recorder, both being connected to the output ofsaid amplifier means.
 26. The colorimeter of claim 20 wherein thecontainer means for holding the sample to be analyzed is a transparentcuvette.
 27. The colorimeter of claim 20 wherein the container means forholding the sample to be analyzed is a fluid flow cell.